The present invention relates to magnetic flux locking methods and apparatus. More particularly, the present invention relates to methods and apparatus for supplying a feedback signal to a feedback coil so as to compensate for variations in the magnetic flux which is guided to a superconductive loop of a superconducting quantum interference device (hereinafter referred to as a SQUID) by an input coil when the SQUID is operated and housed in a casing which is cooled by a refrigerator to a temperature below the critical temperature for superconductivity.
It is known that a SQUID is capable of detecting magnetic flux with extremely high sensitivity. With attention to this characteristic, a SQUID is applied to various apparatus which are used in various technical fields. A SQUID is classified as an rf-SQUID if it has only one Josephson junction (hereinafter referred to as a JJ) and as a dc-SQUID if it has two JJs. The rf-SQUID was generally used in the past years, while the dc-SQUID is being widely used in recent years because two JJs having similar characteristics can be obtained due to improvements in thin film manufacturing engineering in recent years.
FIG. 12 is an electric diagram for explaining the principle of a dc-SQUID flux meter.
The dc-SQUID includes a superconductive loop 71 and two JJs 72 which are provided at predetermined positions on the superconductive loop 71. A bias current is supplied to the opposite positions on the superconductive loop 71 with respect to the JJs 72 by a constant current source 70. An input coil 73, which is interconnected with a pickup coil 74 for detecting the magnetic flux of an object under measurement, is provided at a closed position on the superconductive loop 71. A voltage is output from the opposite positions on the superconductive loop 71 with respect to the JJs 72, the output voltage is transformed by a voltage transformation transformer 75 and then is amplified by an amplifier 76. The amplified voltage is demodulated by a synchronous detector 78 based on the modulated signal output from an oscillator 77, then the demodulated signal is integrated by an integrator 79 so as to output a voltage which is proportional to the exterior magnetic flux. Further, the output signal output from the integrator 79 and the modulated signal output from the oscillator 77 are added by an adder 80. The added signal is converted into a feedback current by a voltage-current converter 81, and the feedback current is supplied to a feedback coil 82 so as to eliminate the exterior magnetic flux detected by the pickup coil 74.
When the dc-SQUID is integrated into a magnetic flux locked loop (hereinafter referred to as a FLL) having the arrangement shown in FIG. 12, the interlinkage magnetic flux of the superconductive loop 71 can be measured by maintaining the magnetic flux at a point having the highest transformation rate of magnetic flux to voltage, because the FLL distinguishes the disadvantage that the interlinkage magnetic flux cannot be measured as it is due to the cyclic alteration of the transformation coefficient of the magnetic flux to the voltage based on the size of the interlinkage magnetic flux (refer to FIG. 6). More particularly, the magnetic flux which has the same size, and polarity which is the reverse of the externally supplied magnetic flux to the superconductive loop 71 through the pickup coil 74 and input coil 73, is fed back by the feedback coil 82 so as to cancel the external magnetic flux. The external magnetic flux can be measured by monitoring the feedback current supplied to the feedback coil 82.
When the FLL having the arrangement above-mentioned is employed, disadvantages arise in that the circuitry scale is enlarged, expensive electrical devices are needed in use, demands for minimizing, and multichannel application are greatly difficult to achieve. Further disadvantages arise in that operability is incorrect and working efficiency is reduced because adjusting operations such as gain adjustment, phase adjustment and the like are needed and the adjusting operations need a fairly long time. Especially, when SQUIDs and FLLs are provided for multichannel application, a disadvantage arises in that multichannel application which is actually achievable is suppressed because the time necessary for adjustment operations is lengthened in proportion to the number of channels. Furthermore, when SQUIDs and FLLs are applied to magnetic flux measurement for living organisms, a computer must receive measured magnetic flux for multichannel performed processings such as estimating a position singled out for an operation and the like. The computer must receive analog values output from corresponding FLLs in a parallel multichannel arrangement for a long time with high speed. The quantity of data which are processed by the computer accordingly becomes extremely great and expensive analog to digital converters (hereinafter referred to as A/D converters) which have high resolution are accordingly employed, thereby a great load is applied to the computer. As a result, disadvantages arise in that multichannel application which is actually achievable is suppressed and a measurement system including SQUIDs and FLLs is expensive.